Communication lines are employed to transmit digital and analog communication signals between remote locations. Because communication lines may be exposed to damaging voltages, such as lightning, electrostatic discharges, power line and other voltages, such lines are often equipped with circuits providing overvoltage protection thereto. Primary overvoltage protection devices include gas discharge tubes which limit large voltages to smaller amplitudes. Secondary protection devices include solid state devices which further limit the damaging voltages to safe levels so as not to damage integrated circuits, semiconductor devices and other electrical components.
Overvoltage protection devices of the semiconductor type are well adapted for protecting down stream communication circuits from surge and other transient voltages which would otherwise damage or destroy the down stream circuits. Transient voltage suppressors (TVS) are readily available for use as overvoltage protection devices. Many semiconductor devices are well adapted for providing overvoltage protection against voltages greater than, for example, one hundred volts. The doping levels in such integrated circuits are easily attainable to provide a breakover or reverse breakdown voltage of such magnitude. These higher breakover voltage devices are generally two-terminal, four layer devices, such as Sidactor® overvoltage protection devices, obtainable under the Teccor brand, from Littelfuse, Des Plaines, Ill. Other two-terminal avalanche junction devices are well adapted for providing protection to low voltage communication lines, such as Ethernet lines.
Overvoltage protection circuits used in conjunction with communication lines may include a semiconductor device providing overvoltage protection functions, or similar devices in combination with other circuits, such as a bridge, for accommodating overvoltages of either polarity. When overvoltage protection devices and corresponding circuits are used to protect low voltage, high speed communication lines, the capacitance of the protection devices and circuits must be very low. Otherwise, the capacitance of the overvoltage protection devices and corresponding circuits can load the communication line to the extent that the bandwidth is limited, thereby compromising the transmission speed of the line.
It is a common practice to provide overvoltage protection devices and circuits in a single package. If, for example, a TVS device is employed with a diode bridge, then the common practice is to solder the TVS device to a lead frame, together with the individual diodes of the bridge, and encapsulate the components together into one package. Sometimes the diode bridge comprises two separate chips, because it is easier to construct one set of diodes in a P-type substrate and the other set of diodes in a separate N-type substrate.
In a typical integrated circuit package employing multiple components, it is a conventional practice to mount the components separately and provide interconnections between the components using wires bonded to the contact pads or terminals of the component devices. The contact pads of one or more of the components can be soldered to the leads of the metallic lead frame. The assembly then undergoes a molding process where a liquified material is injected into a mold which, when solidified, provides mechanical protection to the lead frame and components attached thereto.
Insofar as many users employ hundreds to thousands of communication lines, there is a need for a miniature, cost effective, packaged overvoltage protection device. It can also be seen that a need exists for a single-chip integrated circuit which includes an overvoltage protection device together with a diode bridge fabricated in the same semiconductor substrate. Another need exists for an overvoltage protection integrated circuit providing overvoltage protection functions to a high speed, low voltage communication line. Yet another need exists for an overvoltage protection circuit having provisions for applying a voltage bias thereto for lowering the junction capacitance of the overvoltage protection device.